I'm trying to multiplex small segments of LED Strips.I'm sinking using TLC5940's, i'm sourcing using a ULN2803 driving PNP transistors. (BC577)

I started out at 12v, which neatly gave me about 20mA on each rgb led channel.Since I'm multiplexing I wanted to increase the current going to the LED strips.I did this by increasing the voltage to 18v.

Now, however, the transistors are not 'shutting down' completely, does anyone know how this happens?

I'm using a 2.5kohm resistor between the base and the ULN2803 outputs.

A PNP transistor is effectively a diode from base to emitter ( base cathode ) and pulling too much current from the base WILL destroy the transistor, Short the base to ground and the transistor WILL explode or shut down the power supply, in either case the transistor is toast. ALL PNP transistors need some method of current limiting (A resistor) in the base circuit. If it was an NPN directly pulling the base down, likely the NPN is toast as well.

Bob

--> WA7EMS "The solution of every problem is another problem." -Johann Wolfgang von GoetheI do answer technical questions PM'd to me with whatever is in my clipboard

A PNP transistor is effectively a diode from base to emitter ( base cathode ) and pulling too much current from the base WILL destroy the transistor, Short the base to ground and the transistor WILL explode or shut down the power supply, in either case the transistor is toast. ALL PNP transistors need some method of current limiting (A resistor) in the base circuit. If it was an NPN directly pulling the base down, likely the NPN is toast as well.

Bob

To be honest, i don't quite see how this is relevant..?

Also, when not connecting anything to the transistor except for 12v on the emitter and the collector to the 'anode' of the led strips, i measure about 11v from the collector to ground..